The present invention relates to an electron tube and a method of fabricating the same, and more particularly to a sealed structure between a glass bulb and a glass stem.
In general, an electron tube, especially a color cathode-ray tube has structure such as shown in FIG. 1. An envelope or bulb 1 made of glass material is typically funnel-shaped and comprises a neck tube portion 1a, a funnel portion 1b and a panel portion 1c sealed to the large end of the funnel portion 1b through frit glass material. Within the neck tube 1a is accommodated an electron gun assembly 3a mounted on a usually disk-shaped stem and having a greater 3e attached thereto. A phosphor screen 4 is formed on the inner surface of the panel portion 1c. A shadow mask 5 positioned opposite to the phosphor screen 4 and apart therefrom by a predetermined distance is supported and fixed to the side wall of the panel portion 1c by a supporting member 6. To the end of the shadow mask 5 is attached an electron shield 7 for preventing reflected or secondary electrons due to the over-scanning of an electron beam from impinging upon the phosphor screen 4.
The fabrication of the color cathode-ray tube having the above structure is completed by sealing the stem to an open end of the neck tube 1a after the phosphor screen 4, the shadow mask 5, etc. have been incorporated into the envelope or bulb 1. This sealing process is carried out as follows. As understood from FIG. 2, a stem 3d, on which is mounted an electron gun assembly 3a having a plurality of electrodes fixed on a support of bead glass through leads 3b of the respective electrodes and which includes an exhaust tubulation 3f and outer pins 3c for drawing out the leads 3b, is received by a mount pin 8. The mount pin 8 is thereafter moved and positioned so that the electron gun assembly 3a is accommodated within the neck tube portion 1a of the bulb 1. Then, gas burner flames 9 act on the outer wall of the neck tube 1a around the stem 3d so that the neck tube 1a and the stem 3d are melted for sealing or fusing as shown by solid lines in FIG. 2 and an unnecessary portion or cullet 1a' of the neck tube 1a existing under the melt neck tube portion is burnt off. The burnt-off cullet 1a' remains attached to the mount pin 8 as shown.
For the above-described sealing process there is used an elaborate apparatus having a required high accuracy and the burner flames 9 are usually arranged in a multi-head rotating fashion.
However, the above sealing process involves some drawbacks. First, since the removal of the remaining cullet 1a' is carried out by cracking it, pieces of broken glass are scattered as fine fragments or powder. These finer glass fragments may sometimes enter into the bulb 1 from the exhaust tubulation 3f. Also, the finer glass fragments may frequently enter into the bulb 1 before the sealing between the neck tube 1a and the stem 3d since the steps for mounting the bulb and the stem in a sealing apparatus are carried out within an atmosphere containing the finer glass fragments. As a result, the finer glass fragments may be deposited on the shadow mask 5 accommodated within the bulb 1 to result in the plugging of the mask or the finer glass may be deposited on any locations in the bulb 1 to cause the generation of a discharge within the bulb during the operation of the fabricated tube due to a lowered degree of vacuum resulting in the deterioration of emission ability of a cathode. Second, since the gas burner system cannot provide a localized heating to only those portions of the neck tube 1a and of the stem 3d to be sealed to each other, the electron gun assembly 3a, especially the cathode and the first grid electrode near the stem may be excessively heated so that the electron gun assembly itself and/or the emission characteristic of the cathode is damaged.
The sealed structure is very weak against any mechanical shock since the sealed portion between the neck tube 1a and the stem 3d has an extremely small thickness as compared with the other portions of the neck tube and stem and includes various concentrated strains due to the forced heating by the burner flames.
The sealing process further involves a drawback that a positional relationship between the electron gun assembly 3a and the phosphor screen 4 cannot be attained with high accuracy since the outer wall of the neck tube 1a is heated by the burner flames 9 while the bulb 1 attached to a holder head is rotated both round its own axis and round the center axis of a sealing apparatus.